The invention relates generally to process control, and more particularly to implant dosage control.
Ion implanters, commonly used in semiconductor manufacturing, implant ions into semiconductor substrates to change conductivity of the material in such substrates or in pre-defined regions thereof. Ion implanters generally comprise an ion source for generating an ion beam, a mass analyzer for selecting a particular species of ions from the ion beam, and means to direct the mass-selected ion beam through a vacuum chamber onto a target substrate supported on a substrate holder.
Typically, cross-sectional area of the ion beam at the target substrate is less than the surface area of the substrate which necessitates scanning of the beam over the substrate using a one or two-dimensional scan so that the beam covers the entire surface of the substrate. In the semiconductor manufacturing, it is critical to ensure that for any selected species of ions the wafers are implanted with the correct ion dose and that the dosage is uniform throughout and across the wafer or the part of the wafer targeted to receive the implanted ions. The dosage delivered during an implant process is monitored by measuring beam current using a beam current detector (such as a Faraday cup) positioned behind a wafer. As the beam and the wafer effect movement one relative to the other so that the beam is no longer obstructed by the wafer, the beam can fall on the beam current detector. Where implantation of multiple wafers is concerned, this may be achieved by positioning the beam current detector behind the movable wafer holder with one or more gaps/slits in the holder through which the beam can pass to the beam current detector that is aligned with the general path of the ion beam. Where single wafer implantation occurs, the beam current detector will normally be placed in a fixed position behind the wafer so that the beam impinges on the beam current detector as the wafer is moved out of alignment with the ion beam after each single traverse of the ion beam across the wafer.
Referring to FIG. 1, position voltage measures and corresponding beam current measurements are shown. Conventional beam current detectors measure beam current between transits of the ion beam across a wafer, when the ion beam is not obstructed by a wafer (as shown in point A) Therefore, the true beam current of the ion beam patterning the wafer cannot be obtained using the conventional method.